N I S T Center for Neutron Research
Accomplishments and Opportunities 2001
Neutron Diffraction Contributes to Improving the Fatigue Life of Bridges
Welded attachments to large-scale structures such as bridges can limit the serviceability and prevent effective use of high strength steels. To avoid this, it is desirable to enhance the fatigue resistance of common attachment details such as transverse stiffeners, cover plates, gusset plates and other welded details that, at the weld toe, are often the initiation point for crack growth.
Enhancement of fatigue resistance of welded joints by plastic deformation of the surface and by improvement of weld toe characteristics is well established (Refer to References 1 and 2). It is known that conventional improvement techniques such as grinding, shot peening, air hammer peening, gas tungsten arc re-melting, and use of improved welding consumables can improve fatigue resistance of welded details (Refer to Reference 3). However, these procedures are time-consuming, and can be inefficient and environmentally unfriendly. Ultrasonic Impact Treatment (U I T) offers an alternative means to avoid the negative aspects of other methods (Refer to Reference 4).
The post-weld enhancement of welded details by U I T, developed by Statnikov and co-workers (Refer to Reference 4), involves deformation treatment of welds by impacts at ultrasonic frequency on the weld toe surface. The objectives of the treatment are to introduce beneficial compressive residual stresses at the treated weld toe zones, and to reduce stress concentration by improving the weld toe profile. U I T is claimed to involve a complex combination of strain hardening, reduction in welding strain, relaxation of residual stress, and a reduction in operating stress concentration, thereby achieving a deeper cold-worked metal layer. However, virtually none of the effects ascribed to U I T have been confirmed by appropriate measurements.
Here we highlight neutron diffraction measurements at the N C N R that have contributed new insights into how U I T actually affects residual stresses in the weld region around a full-scale cover plate for a steel girder. In Figure 1 a diagram is shown of a typical I-beam fatigue-tested at the Center for Advanced Technology for Large Scale Structures (A T L S S) at Lehigh University. In the initial phase of the N C N R-Lehigh collaboration, neutron diffraction was used to determine the effect of U I T on a test flange plate by measuring before and after the treatment by U I T. The test plate consisted of a cover plate having a 12.5 mm (0.5 in.) end fillet weld attaching it to a base plate. This kind of cover-plate detail is often used in steel bridges but has a low level of fatigue strength compared to other welded configurations. The geometry and dimensions of the welded detail are the same as in the full-scale fatigue test beams of Figure 1. Both base plate and cover plate are steel (A 572 Gr.50).
Graphics Caption FIGURE 1. Partial details of one of the beam specimens being fatigue-tested at the A T L S S facility. The cover plate location is shown as “W” and “Y” on the right side.
Graphics Caption FIGURE 2. Top view of a flat test plate: cover plate (inner rectangular segment), weld (hatched area), and base plate underneath. Part of measurement mesh is shown by dots.
Graphics Caption FIGURE 3. X-direction residual stresses vs. depth at the weld center. Solid symbols are pre - U I T. Uncertainties are typically ± 20 m p a.
Graphics Caption FIGURE 4. The residual stresses determined by neutron diffraction for the “near-surface” points. Uncertainties due to counting statistics are typically about ± 20 m p a. Solid symbols are before U I T.
Neutron diffraction measurements were made in a mesh, mostly near the weld toe, as indicated in Figure 2. Overall, residual stresses were determined at 30 positions in the weld region, before and after U I T. Gauge volumes of 3 x 3 x 3 mm3 (interior) and 2 x 2 x 2 mm3 (near surface) were used. The unstressed d-spacing, d0, was determined from a small piece of the starting material. The depth dependence of X-direction stresses is shown in Figure 3.
The near-surface residual stresses determined, nominally 1 mm below the actual surface, are shown in Figure 4. Both the anisotropy of stresses around the untreated weld metal, and the additional anisotropy introduced by U I T are seen. Before U I T, the stresses just beneath the weld toe are comparable and highly tensile in the X and Y directions. These stresses reach and exceed the minimum yield stress (315 m p a) of base plate material, whereas those normal to the surface are generally only ≈ 100 m p a tensile. After U I T the X-direction stresses become compressive, changing in magnitude by about 400 m p a. The Y-direction stresses are reduced by ≈ 150 m p a or less but remain in tension except at one point. The normal stresses (Z-direction) are essentially unchanged. The U I T effect on X-direction stresses (Refer to Figure 3) is significant only near the plate surface, but reaches deeper than, for example, shot peening. The depth dependences of the Y- and Z-direction stresses were also obtained.
To our knowledge this is the first determination of the triaxial stress distribution in the vicinity of a weld treated with U I T. Having established the effects of U I T stress-relief on a test flange plate, ongoing neutron diffraction studies in this collaboration are now focusing on the mapping of stress gradients within and near weld joints in sections of actual fatigue-tested I-beams.
References
[1] J. W. Fisher, K. H. Frank, M. A. Hirt, and B. M. McNamee, Effect of Weldments on the Fatigue Strength of Steel Beams, N C R P Report 102, Highway Research Board, Washington, DC, 1970.
[2] J. W. Fisher, P. A. Albrecht, B. T. Yen, D. I. Klingerman, and B. M. McNamee, Fatigue Strength of Steel Beams with Transverse Stiffeners and Attachments, N C H R P Report 147, Highway Research Board, Washington, DC, 1974.
[3] J. W. Fisher, H. Hausammann, M. D. Sullivan, and A. W. Pense, Detection and Repair of Fatigue Damage in Welded Highway Bridges, N C H R P Report 206, Transportation Research Board, Washington, DC, 1979.
[4] Reviewed in Statnikov, Esh, Applications of Operational Ultrasonic Impact Treatment (U I T) Technologies in Production of Welded Joints, Welding in the World 44, 11 (2000).
Authors
H. J. Prask, T. Gnäupel-Herold1 , and V. Luzin2
N I S T Center for Neutron Research
National Institute of Standards and Technology
Gaithersburg, MD 20899-8560
and
1University of Maryland
College Park, MD 20742
2State University of New York
Stony Brook, NY 11794
J. W. Fisher and Xiaohua Cheng
Department of Civil and Environmental Engineering
Lehigh University
Bethlehem, PA 18015-4793